Emenelton: Got it. I sat down at the keyboard (in my office!) and played a dominant chord that resolved down to a minor chord a minor third lower (E.g., D7 to B-7), which is what Nahum was suggesting. It actually sounds reasonably good. Not quite a smooth as a typical V-I, but it seems to work.
Perhaps it works well in this particular turnaround because it has the III chord (after the tonic) as a dominant, which is a nice, bluesy, sound. Yet, the III7 leads naturally to a minor ii chord (G-7 in this case). It's nice to inject another dominant chord in the sequence, thus the Eb7.
Your example of D7 to bmi7 is in the key of A.
That Eb7 in the first ending gives a nice dominant color splash that's outside the key center but resolves in a mainstream manner to the ii7/cm7, so it adds color but fits right in. The gm7 goes down a fifth naturally to the cm7 but the Eb7, in between them, resolves to the same cm7 in a stronger fashion.
Starting on the 13th bar into the 2nd ending the tonal center shifts to the V major.
This progression's tonal center is F:
I sus4 (this is a major sus4 chord with an added 6th.) - VI7 - ii7 - lowered VII dominant.
It does it three times in a row.
Or simply I-VI-ii-V7 (with the flat-VII dominant substituting as a back door dominant for the V7, I refer to this chord as a modal exchange) You can vamp on it all day until you finally decide to take door #1 out from your last Eb7 and hit the ii7-V7-I (cm7-F7-Bb7) in the tonic key to wrap up the song.
Eb7 functions in 3 different ways in Groove Merchant:
as a typical IV7 blues chord (Mike A)
as a borrowed dominate when it leads to the cm7 (Nahum)
as a modal exchange(back door dominant thanks MA) when it resolves up a whole step to F-sus4-add6 (per me)
excuse me I have to go back to the dentist now and have more teeth pulled! LOL